Modern automatic machines such as machine tools and robots, in the course of their work or operation, perform work in which they apply forces to workpieces, or are themselves subject to actions of forces from outside. Consequently, it is necessary for machine tools and robots to detect forces and moments acting on them from outside and to perform control corresponding to these forces and moments. For control corresponding to external forces and moments to be carried out properly, it is necessary for the forces and moments acting from outside to be detected exactly.
In this connection, various types of multi-axis force sensors to be used as force-sensing sensors and man-machine interfaces have been proposed. Generally, force sensors can be classified, according to the detection method that they use, as either elastic-type force sensors, which detect a deformation proportional to a force, or force-balance-type force sensors, which measure a force by balancing it with a known force. As a principle structure, force sensors generally have multiple strain resistance devices provided on a distorting body part which deforms elastically in correspondence with external forces.
With this structure, when an external force acts on a distorting body part of the multi-axis force sensor, electrical signals corresponding to degrees of deformation of the distorting body part are outputted from the strain resistance devices. On the basis of these electrical signals it is possible to detect two or more force or moment components acting on the distorting body part.
To keep up with size reductions of devices equipped with multi-axis force sensors, size reductions of multi-axis force sensors themselves are sought. Accordingly, there is an increasing need for multi-axis force sensors which have good sensitivity and high precision while being small.
A typical multi-axis force sensor is the six-axis force sensor. The six-axis force sensor is a force sensor of the elastic type mentioned above, and has multiple strain resistance devices on a distorting body part. The six-axis force sensor divides an external force into axial stress components (forces Fx, Fy, Fz) in the axis directions and torque components (torques Mx, My, Mz) about the axis directions of three orthogonal coordinate axes (an X-axis, a Y-axis, a Z-axis), and detects it as six axis components.
A first example of a multi-axis force sensor in related art is the ‘Multiple Force Component Load Cell’ disclosed in Japanese Patent Publication (JP-B) No. 63-61609 published on Nov. 29, 1988 (corresponding to U.S. Pat. No. 4,448,083). This document discloses a six-axis force sensor. This six-axis force sensor has a construction wherein multiple strain gauges are affixed to a distorting body having a solid (three-dimensional) structure. This sensor is a six-component force sensor and has a structure such that mutual interference among the six force components in different axes being detected is reduced. The mutual interference will be sometimes referred to as “different axis interference”. The six-axis force sensor is made up of a central force-receiving part, a fixed annular part around this, and between these, four T-shaped connecting parts equally spaced around the axis of the force-receiving part. The strain gauges are affixed to low-rigidity portions of the beams of the T-shaped connecting parts.
With this structure wherein strain gauges are affixed to a distorting body, size reduction is limited; manufacturing reproducibility is poor and dispersion arises among units; and also problems such as peeling of the affixing layer arise due to repeated shock stresses and thermal stresses. When peeling of the affixing layer occurs, the measuring precision deteriorates. Alignment deviation also causes the measuring precision to deteriorate. The problem arises that it is difficult to make the mounting positions accurate enough to ensure good detection accuracy.
A second example of a multi-axis force sensor in related art is the ‘Two-or-more-Component Force-Detecting Device’ disclosed in Japanese Patent No. 2746298 published on May 6, 1998. In the multi-axis force sensor disclosed in this document, multiple strain resistance devices are fabricated on a substrate using semiconductor manufacturing process technology, and a strain gauge element is assembled integrally to a distorting body part. The substrate is made up of a peripheral part and a central part. According to this document, the problems of the multi-axis force sensor of the first related art mentioned above can be resolved, the precision of the fabrication process can be raised, the reproducibility of fabrication can be made good, and the multi-axis force sensor can be reduced in size. However, with this sensor, there is a high probability of mutual interference arising among the six axis components detected.
A third example of a multi-axis force sensor of related art is the ‘Contact Force Sensor’ disclosed in Japanese Patent Publication (JP-B) No. 07-93445. In this contact force sensor also, piezoelectric sensors made by forming resistance devices on one side of an annular structural body made of a semiconductor are used, and semiconductor manufacturing process technology is utilized.
Of the multi-axis force sensors of these first through third examples of related art, whereas in the first multi-axis force sensor strain resistance devices (strain gauges) are affixed as external elements, in the second and third multi-axis force sensors, strain resistance devices are formed integrally on a semiconductor substrate by utilizing semiconductor device manufacturing process technology. The second and third multi-axis force sensors have the merit that they make it possible to resolve the problems associated with the first multi-axis force sensor.
However, related art multi-axis force sensors fabricated using semiconductor device manufacturing process technology have had the characteristic structurally that, when an attempt is made to detect a force or moment on each of three orthogonal axes, the whole substrate distorts isotropically in correspondence with the applied force or moment, and have also had the problem that the disposition of the multiple strain resistance devices on the substrate is not optimal and an external force acting on the distorting body part cannot be separated into components with good precision.
That is, in six-axis force sensors, there has been the problem that for example when an external force is applied so that only an axial stress component Fx arises, stresses arise and outputs are produced in connection with components other than Fx, which should properly be 0. There has been the problem that it is difficult to separate an external force applied from an unknown direction into individual components with good precision. The electrical signal components outputted from the resistance devices corresponding to the respective axis components superpose onto the other axes, and the measuring sensitivity of the axis components of force or moment decreases.
The problem of not being able to separate the axis components (forces and moments) of an external force acting on the distorting body part in a six-axis force sensor is known as the problem of ‘other axis interference’. This problem of other axis interference is one which cannot be ignored from the point of view of realizing a practical six-axis force sensor.
The problem of other axis interference in a six-axis force sensor will now be explained more specifically, from a mathematical point of view, using equations.
In a six-axis force sensor, as mentioned above, as six axis components pertaining respectively to an X-axis, a Y-axis and a Z-axis, forces Fx, Fy and Fz and moments Mx, My and Mz are detected. The six-axis force sensor outputs six signals Sig1 to Sig6 (‘computed resistance change proportions’) using resistance changes of strain resistance devices provided on a distorting body part and on the basis of an external computing part. These six output signals Sig1 to Sig6 are associated with the six axis components Fx, Fy, Fz, Mx, My, Mz using 6×6 matrix elements obtained by finding in advance the size (electrical change proportion) of each signal with respect to an input made by applying as an external force a single axis component only.
For a six-axis force sensor, the six axis components Fx, Fy, Fz, Mx, My, Mz will be written respectively as F1, F2, F3, F4, F5, F6 (generally, ‘Fi:i=1–6’). The above-mentioned six output signals Sig1 to Sig6 will be written S1, S2, S3, S4, S5, S6 (generally, ‘Si:i=1–6’).
Between Si and Fi above, expressed with a matrix equation (the symbols ‘( )’ in the equation indicating matrices), the following relationship holds.(Si)=(mij)×(Fj) (j=1–6)  (101)
That is, the equation (101) has the following meaning:                S1=m11.F1+m12.F2+m13.F3+m14.F4+m15.F5+m16.F6        S2=m21.F1+m22.F2+m23.F3+m24.F4+m25.F5+m26.F6        . . . (abbreviated)        S6=m61.F1+m62.F2+m63.F3+m64.F4+m65.F5+m66.F6        
In equation (101), by finding in advance the computed respective resistance change proportions S1–S6 corresponding to the input of a single component only, it is possible to obtain the matrix elements mij of the matrix (mij). By calculating the inverse matrix (mij)−1 of the obtained matrix (mij), the following equation is obtained:(Fi)=(mij)−1×(Sj)=(m′ij)×(Sj)  (102)
The equation (102) has the following meaning:                F1=m′11.S1+m′12.S2+m′13.S3+m′14.S4+m′15.S5+m′16.S6        F2=m′21.S1+m′22.S2+m′23.S3+m′24.S4+m′25.S5+m′26.S6        . . . (abbreviated)        F6=m′61.S1+m′62.S2+m′63.S3+m′64.S4+m′65.S5+m′66.S6        
In the above equation (102), “m′ij” is a matrix element of the 6×6 inverse matrix (mij)−1.
From the above equation (102), on the basis of the computed resistance change proportions (S1–S6) obtained from the resistance change proportions of the semiconductor strain resistance devices, it is possible to calculate the six axis components F1–F6 (the forces and moments of each axis direction).
In equations (101) and (102) above, if the values of the matrix elements mij, m′ij are all large, then for example when the computed resistance change proportions Si fluctuate due to the superposition of noise, that influence appears in the measurement values of F1–F6. That is, when there is an input consisting of a single component only as an external force, although the inputs of the other components are “0”, there is a high probability of the phenomenon arising of the measurement results not being “0” due to disturbances such as noise.
As mentioned above, the obtained measured value of one of the six axis components, that is, forces or moments, fluctuating as a result of a force or moment of another axis is defined as ‘other axis interference occurring’.
Ideally, in the matrix (m′ij), the non-diagonal elements, i.e. the elements other than the diagonal elements m′11, m′22, m′33, m′44, m′55 and m′66, should be “0”. In this case, the above-mentioned equation (102) becomes as follows:                F1=m′11.S1        F2=m′22.S2        . . . (abbreviated)        F6=m′66.S6        
If this relationship holds, the calculation becomes extremely simple, and other axis interference can be prevented.
In practice, even if the non-diagonal elements cannot be made “0”, if the values of the non-diagonal elements can be made extremely small compared to the diagonal elements, the problem of other axis interference can be reduced.
However, with the second and third multi-axis force sensors of related art mentioned above, because the whole substrate distorts and insufficient consideration has been given to the suitability of the disposition pattern of the semiconductor strain resistance devices, the non-diagonal elements of the matrix (m′ij) cannot be made “0” or made sufficiently small compared with the diagonal elements, and the probability of other axis interference occurring is high. Also, with the multi-axis force sensors mentioned above, due to other axis interference readily occurring, noise caused by unexpected disturbances and the like superposes on the electrical signals from the strain resistance devices, and consequently there is a high risk of measurement results fluctuating greatly with other axis interference as the cause. Consequently, when the second and third multi-axis force sensors mentioned above are used on a robot or the like, depending on their installation conditions, other axis interference causes their measured values to fluctuate, and when they are made general-purpose parts there are problems with reproducibility and robustness.
A fourth example of a multi-axis force sensor of related art is the ‘Micro-Manipulator Having Force Sensor’ disclosed in Japanese Patent Laid-open Publication (JP-A) No. 11-333765. The force sensor disclosed in this document is fabricated using semiconductor manufacturing process technology, like the second and third related art examples mentioned above, and a three-component force sensor made up of a base and a central thick part and thin supporting parts connecting thereto and having strain sensors provided on the supporting part is shown.
To resolve the above-mentioned shortcomings of the multi-axis force sensors of the aforementioned second and third related art examples using semiconductor manufacturing process technology, in this fourth related art example, a structure is proposed wherein components of strain are separated axis by axis. However, although this structure achieves a slight improvement compared to other related art, it is a construction for performing detection of forces in the directions of three axes (X-axis, Y-axis, Z-axis), and when it is used as a six-axis force sensor its component-separating capability is inadequate and it cannot resolve the problem of other axis interference.
A further, fifth example of a multi-axis force sensor of related art is the ‘Sensor’ disclosed in Japanese Patent Publication (JP-B) No. 05-75055. This sensor is formed using a semiconductor substrate and has a central supporting body, a peripheral supporting body, and a plurality of connecting parts (beams) connecting these. According to FIG. 1 of this document, a resistance film pattern made up of multiple resistance devices is formed integrally with the surfaces of two predetermined connecting parts (beams) by film-forming technology. Because the connecting parts are parts of the semiconductor substrate, they have a thin plate shape. When the sensor receives a force on the central supporting body, the semiconductor substrate itself bends as a whole, and six force components are taken out by the multiple resistance devices provided on the connecting parts. In the sensor of this related art example also, there is a possibility of mutual interference among the detected six force components.
Another issue addressed by the invention will now be discussed. A six-axis force sensor fabricated using semiconductor manufacturing process technology contributes to sensor device size reduction. To make a six-axis force sensor small, the semiconductor substrate becomes small and becomes thin. In a six-axis force sensor formed using a semiconductor substrate, the semiconductor substrate itself functions as a distorting body. Consequently, there is a limit on the range of forces which can be measured, which depends on the basic strength of the semiconductor substrate. From the point of view of practical application, there is a need for a sensor to be designed so that this limit is not problematic, and for the measurement range to be raised to widen the range of applications.